Integrated Circuits (ICs) generally comprise many semiconductor features, such as transistors, formed on a semiconductor substrate. The patterns used to form the devices may be defined using a process known as photolithography. Using photolithography, light is shone through a pattern on a mask, transferring the pattern to a layer of photoresist on the semiconductor substrate. The photoresist can then be developed, removing the exposed photoresist and leaving the pattern on the substrate. Various other techniques, such as ion implantation, etching, etc. can then be performed to form the individual devices.
To increase the speed of ICs such as microprocessors, more and more transistors are added to the ICs. Therefore, the size of the individual devices must be reduced. One way to reduce the size of individual features is to use short wavelength light during the photolithography process. According to Raleigh's Law (R=k*λ/NA, where k is a constant, and NA=Numerical Aperture, and R is the resolution of features), a reduction in the wavelength of the light proportionately reduces the size of printed features.
Extreme ultraviolet (EUV) light (13.5 nm) is now being used to print very small semiconductor features. For example, EUV can be used to print isolated features that are 15–20 nanometers (nm) in length, and nested features and group structures that have 50 nm line space. EUV lithography is targeted to meet the requirements of a 50 nm half-pitch, where pitch is equal to line plus feature size.
Since EUV light has such a short wavelength, it is easily absorbed, even by air. Therefore, for EUV photolithography to be viable, mirrors are used for focusing rather than lenses. The mirrors used for focusing need to have a high reflectivity since the transmission rate of EUV light is low.
EUV photons can be generated by creating a dense plasma. One way to generate the photons is to project a laser beam into a target (droplet, or filament) which produces the plasma, heating it, and thereby exciting the atoms. When the excited atoms return to a stable state, photons of a certain energy, and thereby a certain wavelength, are emitted. The target may be, for example, Xenon, Tin, or Lithium. Another way to produce EUV photons is to use an arc lamp producing a high temperature between two electrodes having the plasma between the two electrodes.
Typical EUV optics include a set of mirrors to focus light generated by a light source, an obscuration in front of the light source to block debris generated by the light source, a condenser, and a reticle. The EUV light source generates EUV photons, which are reflected by the mirrors and directed through the condenser. The condenser typically includes a collimator to collimate the incoming light. The light is then directed through a reticle, which includes the pattern to be lithographed on a substrate.
The set of mirrors typically comprises an array of mirrors including two separate segments. The light is reflected off of the first segment, and then off of the second segment to focus the light. Each segment absorbs a percentage of the generated light. For example, if the reflectivity of each segment is 70%, when the light is reflected by both segments, only 49% of the incident light is transmitted by the mirrors.
The obscuration is typically a foil disc placed in front of the EUV light source to block debris created by Brownian motion. The obscuration also absorbs a large portion of the light generated by the EUV light source. Further, the collimator also absorbs a portion of the light.